Application of magnetic particles in a fluid suspension, or in dry form, to a magnetized workpiece to detect flaws is a technique known in the art. According to that technique, the workpiece is magnetized, and then the magnetic particles are applied. Discontinuities or inhomogeneities in the workpiece which lie substantially transverse to the direction of the principal magnetic field cause the occurrence of localized leakage fields which capture some of the magnetic particles. Collections of these particles held by the leakage fields form patterns which reveal the existence and locations of the discontinuities and inhomogeneities.
In a more recent development magnetic particles have been combined with a fluorescent pigment to aid in detection of discontinuity- or inhomogeneity-caused patterns after the magnetic particles have been applied to the magnetized workpiece. A further development is the encapsulation of adherent fluorescent pigment and magnetic particles in a coating of film-forming resin, as disclosed in U.S. Pat. No. 3,485,758, naming as inventors James S. Borucki and Paul Kenneth Borrows, granted Dec. 23, 1969, and assigned to Magnaflux Corporation.
The magnetic particle inspection method described above is employed commercially to detect flaws in relatively large ferromagnetic bodies. One of the principal areas of its utilization is in the inspection of steel billets. Flaws such as seams in those billets result from any of a large number of causes, some mechanical and some metallurgical. Typical examples are removal of metal from the steel surface by scaring or scaling; formation of strings of non-metallic inclusions; formation of metallic inclusions which have a different permeability from the parent metal; the occurrence of overfill or underfill in the rolls during hot or cold working operations; and formation of cold shuts due to an overlapping of the metal resulting from splashing in the mold during ingot formation. It is particularly important to locate seams which are longitudinal discontinuities in such billets; those seams appear as light lines in the surface of the steel. They are normally closed tight enough so that no actual opening can be visually detected, thus making magnetic particle inspection necessary.
However, several distinct problems and disadvantages accompany use of conventional magnetic particle inspection techniques, especially as regards steel billets. Those billets are normally inspected in a continuous process in which each billet travels at relatively high velocity (for example, about 120 feet per minute) through a magnetizing field, and then through an applicator station where fluorescent-type magnetic particles are applied.
Conventional methods for application of magnetic particles in dry form onto the billet have the inherent shortcoming that under some conditions the magnetic particles do not come to complete equilibrium on the magnetized billet in the short time allotted due to the high speed of handling; that precludes formation of patterns corresponding to smaller inhomogeneities and discontinuities in the steel billet and prevents their detection. A further requirement of dry application which can be disadvantageous is that the billets must be pre-heated at least to room temperature. Nevertheless, for some jobs dry application is an acceptable and convenient technique.
But, even when they might otherwise be acceptable, conventional dry application techniques are not always satisfactory. That state of affairs results from the fact that a dry magnetic particle bath normally contains a binder agent which fixes the magnetic particles in the bath to the workpiece. The binder agents ordinarily employed - such as resins, waxes, and even in some instances sodium silicate - are not insubstantial in their contribution to the overall cost of formulating the bath, and difficulties in obtaining some of those agents can also add expense and inconvenience. Furthermore, a number of conventionally employed binders are incompatible with post-inspection practices, and are difficult to remove from the workpieces when the need to preserve the flaw-indications formed by clustered magnetic particles has passed.
The wet method eliminates some of the problems attendant on dry application and thus is more suitable for use under conditions which make dry bath application less desirable. But the wet method also has certain drawbacks under some conditions encountered in commercial operation.
While the use of oil for suspending magnetic particles has the advantage of dissolving any oily film on the surface of the workpiece to be tested, the oil suspension impedes drying, and thus affixation, of the magnetic particles on the billet. Throughout the inspection process, the billet is likely to be handled roughly with the result that the collections of magnetic particles at seams and other flaws may be disturbed or even destroyed. Further, the oil bath can present a potential fire hazard depending on the means used to magnetize the workpiece.
Thus, in the testing of larger workpieces, and especially steel billets, utilization of a water bath is favorable due to lower costs and the complete elimination of bath flammability. That bath conventionally contains a wetting agent--for example a non-ionic wetting agent such as ethoxylated nonylphenol--to aid in dispersing the magnetic particles on the billet so that the bath can spread evenly over its entire surface. The bath frequently contains an adhesive, such as a resin, wax or sodium silicate, which acts as a binder to fix the magnetic particles on the workpiece upon drying or evaporation of the bath liquid. The adhesive minimizes the chance that magnetically formed indications will be disturbed by jostling of the workpiece during handling prior to inspection.
However, conventional water baths have the disadvantage that they cannot be used when the temperature drops below freezing unless the bath and/or workpiece are pre-heated, a measure which results in processing difficulties and economic loss. Use of anti-freeze components in the water bath is generally not feasible because the quantities needed to be effective would raise the viscosity of the bath above the maximum allowable. High bath-viscosity slows the movement of particles under the influence of the magnetic field. At viscosities above about five Centistokes, the movement of magnetic particles in the bath is sufficiently retarded to have a definite effect in reducing the build-up, and therefore the visibility, of patterns of magnetic particles at small discontinuities in the workpiece.
Indeed, the anti-freeze component ethylene glycol when actually employed by itself has exhibited just such disadvantageous behavior, becoming viscous at low temperatures and decreasing the mobility of the magnetic particles in suspension. Moreover, ethylene glycol by itself impedes drying of the bath liquid after application on the workpiece and thus prevents affixation of the magnetic particles on the workpiece. Rather, a slushy film is formed on the workpiece which is often disadvantageously removed during handling of the workpiece prior to its inspection for flaws. Yet another disadvantage of ethylene glycol is that it insolubilizes the adhesive. (Other known anti-freeze components such as alcohol and sodium chloride, if present in more than 8% by volume, will also precipitate the adhesive out of solution.)
It can thus be readily appreciated that provision of magnetic particle bath, and inspection method, embodiments which confer on the art the advantages of magnetic particle testing using either a dry or water-base bath, but eliminate the previously discussed problems, would be a highly desirable advance over the current state of technology.